1. Field of the Invention
The present invention relates to a communication system which transmits coded speech information, and more particularly, to a speech signal transmission system and speech signal transmission method for packetizing and transmitting parameters which are coded using CELP type speech coding.
2. Description of the Related Art
Conventionally, in packet communication represented by Internet communication, when, for example, packets are lost in a transmission channel and the decoder side cannot receive coded information, packet loss concealment processing is generally carried out. As one of techniques handling such a packet loss, a scheme shown in FIG. 1 is known.
The transmitting side carries out processing on the digital speech signal input in units of a frame of several tens of ms. In FIG. 1, F(n) denotes coded data of an nth frame and P(n) denotes an nth payload packet.
FIG. 1 shows how coded data of two consecutive frames are multiplexed into one packet and transmitted from the transmitting side to the receiving side. Since the frames multiplexed into the same packet are shifted by one frame at a time, coded data of each frame is transmitted twice from the transmitting side to receiving side using different packets.
After demultiplexing of packets, the receiving side carries out decoding processing using coded data of one of the two received frames (a lower frame number in the figure). When there is no packet loss, all coded data which has been superimposed and transmitted becomes useless, and since two frames are multiplexed together, transmission delay increases by one frame compared to the case where transmission is performed frame by frame.
However, even when there is a packet loss, if only one packet is lost as shown in FIG. 2, it is possible to use coded data included in the packet received immediately before and therefore there is no influence of the error (packet loss).
Such a transmission method is disclosed in IETF standard RFC3267, etc. However, if two or more packets are consecutively lost, there are frames which lose coded data, and therefore it is necessary for a decoder to carry out frame loss concealing processing. An example of frame loss concealing processing is a method described in 3GPP3GTS26-091.
However, packet (or frame) loss concealing processing is carried out independently on the decoder side using coded information already received in the past, and therefore if the coding processing has been performed on the coder side using past coded information, influences of the packet loss propagate not only to the lost part but also to sections following the lost part and may drastically deteriorate the quality of decoded speech.
For example, when a CELP (Code Excited Linear Prediction) scheme is used as a speech coding scheme, speech coding/decoding processing is carried out using a past decoded/driven excitation signal, and therefore if processing on a lost frame causes different decoding excitation signals to be synthesized for the coder and decoder, the internal states of the coder and decoder may not match for a while thereafter drastically deteriorating the quality of the decoded speech.
Therefore, the conventional speech coding method has a problem that when consecutive packet losses occur, the quality of decoded speech drastically deteriorates. The above described conventional method has another problem of requiring an additional transmission delay corresponding to one frame.